Mechanisms of shoreline erosion in a back-bay environment, Cape Carancahua, Texas

Abstract

Because the back-bays along the coast of Texas are becoming increasingly populated, erosion problems associated with development along this shoreline are becoming more common. Past and present efforts to understand and control shoreline erosion, here and elsewhere, have normally focused on topics such as wave erosion, longshore drift, sediment starvation, tidal currents, wave reflection and scour caused by retaining walls, and loss of protective wetlands. However, because this back-bay area is estuarine and is relatively low in wave energy, it is not affected by the problems more typical of the open coast. At this location, there are other more effective mechanisms of erosion such as headward and downward gully erosion, piping, circular-arc and planar slope failures, and some wave erosion and scour. The operation of these mechanisms has been made more effective because human alterations have increased pore-water pressures, increased run-off velocities, increased run-off volumes, and enhanced other variables that increase the rate of sediment removal. Examples of these alterations include the removal of protective marine vegetation, removal of upland coastal vegetation, development of wave reflective sea-walls, lawn-grass watering, installation of septic systems, and dredging at the bay inlet. Interestingly, the main controls behind all of these mechanisms were found to be the geotechnical properties of the clayey shore and coastal materials. The clays at the site were tested and found to be highly susceptible to shrink-sweff, liquefaction, and dispersion. The soil types identified were able to be correlated to specific failure mechanisms. Of course, other common, yet minor factors were also found to influence erosion. Because soil-mechanics were found to be the dominating erosive factor rather than wave erosion, a future need has been identified to evaluate geotechnical properties of the shore and coastal materials in the search for effective solutions to protect these types of eroding back-bay shorelines.